can you use wd40 as lubricant for a propane adapter?

Absolutely not. The chemicals in WD 40 eat away at the hop-up components of your gun.
Eventhough it's being used on the adapter, the propane going through the adapter will collect some of the WD40 and transport it into the gun's insides.

Silicon oil and silicon lube. Preferably anything with 90% of more silicon with as little petroleum as possible (preferably none).